This invention relates to combination battery chargers and power supplies, and more particularly to battery chargers which vary the output voltage with temperature.
In outdoor electronic system installations, e.g., outside plant telecommunications equipment, batteries are often used to provide needed power in the event that local primary power (A.C.) to that installation fails. These batteries must provide power to the installation over wide temperature extremes for a predetermined period of time, typically eight hours. At low temperatures, battery capacity is severely diminished. For example, unsealed lead-acid (secondary) batteries only have 50% capacity at -20.degree. C. (relative to the capacity of the battery at 20.degree. C.). A new type of battery, a sealed lead-acid battery having pure lead electrodes, hereinafter referred to as a sealed lead-acid battery, as manufactured by Gates Energy Products, Inc., Denver, Colorado, has additional capacity at low temperatures, typically 50% at -40.degree. C. While this battery allows operation of the electronic equipment at low temperatures, a widely varying charge (float) voltage is needed to recharge the battery. For example, at -40.degree. C., 67 volts is needed to maintain a charge on the batteries, but at 25.degree. C., the float voltage falls to 56 volts. Additionally, the float voltage does not vary linearly with temperature, i.e., the temperature coefficient of the required float voltage is not constant but varies with temperature. Using a constant float voltage designed to charge the battery at 25.degree. C. (the power supply having no temperature coefficient) the battery life is shortened due to the battery being overcharged at high temperatures and undercharged at low temperatures. To overcome the nonlinear float voltage variation with temperature characteristics, two approaches are used in the prior art: constant current charging and "gear shift" charging. Constant current "trickle" charging charges the batteries by keeping a low charging current flowing into the battery regardless of the temperature or battery float voltage. Although the temperature dependence of the float voltage is overcome with this approach, very long periods are required to recharge a depleted battery. Therefore, if the battery is called upon to provide power during this charging interval, full capacity would not be available and the installation may not be powered for the full predetermined period. This is especially critical in areas having an unreliable primary power source. The "gear shift" charging approach switches from a high charge current to a low charge current if a predetermined rate of change of the battery voltage is exceeded regardless of temperature. With this approach, the time period required to charge the battery is reduced, but this reduced time period is far from optimal for the quickest recharge while having long battery life. In addition, constant current and "gear shift" type power supplies are unsuitable for powering loads with widely varying power requirements due to wide voltage fluctuations across the load.